Orbital Welding of Titanium Pipe for Troll and Heidrun Offshore Production Platforms

Orbital Welding of Titanium Pipe for Troll and Heidrun Offshore Production Platforms

Introduction

Fig. 1 - Troll

Norwegian Contractors (NC), the world's leading supplier of concrete offshore structures, is currently in production on Norske Shell's Troll gravity-base structure (GBS) and the concrete hull of Conoco's Heidrun tension-leg platform (TLP). These projects, which represent the latest technological advances in production platform engineering for the recovery of oil and gas from offshore sites, will incorporate the use of titanium pipe for fire water protection systems, ballast and drain systems, and for pump caissons. The use of titanium for these piping systems is intended to eliminate the need for inspection and repair over the projected 70 year lifetime of the Troll platform and the 50 year estimated lifetime of the Heidrun platform. The Troll project will incorporate about 400 tons of titanium while, at present, 300 tons have been used on the Heidrun project. Fiberglass, another advanced piping material, will be used for low-pressure piping applications in areas which are easily accessible for inspection and repairs.

Space on the platforms is severely limited, and orbital welding was considered at the planning phase of the projects resulting in the platforms being designed with orbital GTA welding equipment in mind in order to optimize the available space.

The clearance for the Arc Machines Model 15 weld head is less than half that required for hand welding; 3-1/2 inches or 92mm for the M-15 vs. 19 inches, which is about 1/2 meter, for hand welding. In keeping with the latest technological advances in evidence on these platforms, orbital pipe welding technology will be used for all the field welds of titanium piping on the platforms.

At its completion, which is scheduled for July of 1995, Troll will be the world's tallest offshore concrete platform. Construction of the Troll platform was begun at one of the two drydocks at NC's yard at Stavanger, Norway, in July of 1991. In late June of 1993, the substructure was towed to Gransfjorden near NC's yard for the intermediate phase of construction. The final work on the structure and deck mating will take place at a deep water site at Vats, which is about 30 miles north of Stavanger. Installation of the completed platform on the field is scheduled for July, 1995.

The Heidrun platform will be the world's first tension-leg platform with a concrete hull. The use of titanium piping on the Heidrun platform is in keeping with design considerations for making the platform as light as possible in order for it to float. The bottom section of the Heidrun hull was under construction in drydock at Stavanger during the summer of 1993 and will be moved to the Gandsfjorden site. Installation on the field is scheduled for July, 1995.

Use of Titanium

Titanium is light and strong and, because of its exceptional corrosion resistance in a seawater environment, will last the lifetime of the platforms. When the field is no longer productive, the platforms will be dismantled and the titanium piping can be recycled. Titanium is stronger and lighter and requires less maintenance than materials such as copper-nickel or 6 moly (super austenitic stainless steel) which have been used for similar piping applications in the past. Its resistance to corrosion makes it suitable for use in areas where inspection is not feasible; for example, some of the titanium piping used for carrying cables will actually be embedded in concrete. Once the commitment was made to use titanium piping for the platforms, NC, with little previous experience in welding titanium, had to gear up to learn about the special handling requirements of titanium and how to weld it successfully.

Welder Training

Fig. 3 - AMI Model 15 weld head mounted on a very large titanium pipe. Note the trailing shield behind the torch which provides gas coverage to the weld and HAZ to protect it from oxidation.

Norwegian Contractors have become the largest user of titanium pipe in Europe. Initially, when they were first faced with welding titanium, they were somewhat in awe of this metal. However, NC set up a school on their site in Stavanger for training manual welders to weld titanium and selected only skilled certified TIG welders for this training. The course lasts two weeks and provides extensive welding theory followed by "hands on" practical experience in the welding of titanium. The number of welders qualified to weld titanium is increasing all the time as more welders pass through school. Training for the orbital GTA welding equipment was provided by AMI Sales Representative, Jorgen Levesen of Teamtrade A/S of Stavanger, Norway, and Francois Exelmans from AMI's European office in Gland, Switzerland, who spent 2 weeks training welders at the NC facilities to operate the orbital equipment. Only welders certified for hand welding of titanium were selected for the orbital welder training.

Orbital Welding Equipment

Two AMI Model 227 microprocessor-controlled power supplies were purchased to operate the Model 9-7500 fusion weld head and the Model 15 full-function pipe weld head.

The 9-7500 weld head was used for fusion butt welds of thin-walled (thinnest was 2.7mm) pipe up to 4 inches in diameter. The Model 15 was used for welding larger piping which had diameters up to 42 inches and wall thicknesses up to 20mm.

The Model 15 weld head has the capability of adding wire to the weld, it provides oscillation of the torch and a constant arc gap via the automatic arc voltage control.

Weld procedures were developed for the different pipe sizes and stored in the memory of the Model 227 power supply. Once a program for a particular size pipe has been worked out, it is a simple matter to recall the program or, in the case of multi-pass welds, any particular pass of the program, and execute it without affecting any of the other programs in the Model 227's library.

GTAW is the most widely used process for the welding of titanium because it provides superior control of the arc and heat input and the obligatory inert gas shielding of the weld puddle. Orbital welding offers the advantages of the TIG process, but also provides even more precise control of those weld parameters which control the heat input to the weld.

Those weld parameters include amperage, arc voltage control, travel speed, pulsation, wire feed, etc. This level of control makes it practical to get a very high degree of consistency from weld joint to weld joint at a very high standard of quality. Although the titanium pipe did not arrive at the site until the second week of training, two acceptable orbital pipe welds were made for weld qualification by the end of the orbital welder training class.

Three different methods were used for welding of the titanium piping for the platforms. In addition to the orbital and manual GTA welds, NC has a plasma welding machine for making downhand welds on large pipe with wall thicknesses up to 24mm which is nearly an inch thick. This was used in NC's workshop for prefabrication of pipe that could be lifted into place and rotated while the electrode remained stationary. The height of the table was adjustable for different pipe sizes. Its use was limited by the geometry of the assemblies; while a 48m riser could be easily lifted into position for welding, subassemblies with hubs or branches which would resist rotation had to be done orbitally or by hand.

It was most economical to prefab as much of the piping as possible. Approximately 80% of the work has been prefabricated and about 20% were field welds.

Special Considerations for Welding Titanium

Fig. 5 - Passageway for X-ray testing of welded pipe.

Titanium requires a higher degree of cleanliness than stainless steel and greater care in purging to avoid oxidation of the weld and heat-affected-zone (HAZ) at temperatures of about 426 degrees C (800 degrees F). A well purged weld will have a silvery metallic appearance. Oxidation is indicated by the presence of discoloration that ranges from pale straw to blue or grey to whitish. Oxidation may cause embrittlement of the material, so bend tests, tensile tests and/or hardness testing of the weld are advisable in order to establish purging guidelines.

For titanium welding, in addition to the shielding gas applied via the ceramic cup around the torch which protects the weld puddle and the backpurge applied to the inside (ID) of the pipe, a trailing shield is usually recommended. NC used special trailing shields made from sintered metal, permeable to the inert gas, which provides coverage of the weld and HAZ behind the torch for a sufficient distance to allow the metal to cool to below 426 degrees C before being exposed to atmosphere. The shoes of the trailing shields were of various lengths to accommodate the different pipe sizes. Argon gas, grade 4.6 (which is 99.996% pure) was used for purging. Purge dams with a rubber seal that fit under the weld joint were used to limit the ID volume to be purged, and the argon flow rates for the weld head, pipe ID, and trailing shields were carefully monitored.

In order to meet the cleanliness standards which are essential for the successful welding of titanium, NC set up their workshop as a special clean area for the exclusive purpose of welding titanium pipe. The clean area was sealed off so that all visitors and welders could only enter the area via the foreman's office. The wearing of booties similar to those worn in cleanrooms is enforced. The floor is swept daily and vacuumed and washed weekly. Special vacuum hoods are used to take away any welding fumes. Instead of carrying welded pipe out the door for radiographic testing, which would result in contamination of the welding area, a passageway in the wall is used to pass the pipe directly into and out of the concrete bunker used for radiographic testing, which is adjacent to the welding hall. The repeatability of orbital welds depends on consistent and uniform end-preparation.

For shop fabrication, a lathe was used to produce a modified J preparation with a land for orbital pipe welding requiring the use of filler material.

End-preparation tools are being used for end-preparation for the field welds. The weld joints are cleaned by grinding with a rotating file and wiped with acetone just before welding to remove all traces of dirt and grease. After welding, the welded assemblies are carefully wrapped in aluminum foil for protection from damage and contamination during transport to the field.

Welding Specifications and Quality Control

Fig. 6 - Heidrun

The piping material used for the platform piping was Grade 2 Titanium with Grade 1 used for filler wire. Other than the requirements for purging and cleanliness, this material does not present any particular difficulty in welding. The puddle is quite liquid and the NC welders found it advantageous to start the weld at the 12 o'clock position to gain better control of the puddle. Titanium fusion welds above a certain wall thickness tend to appear somewhat concave at the top of welds done in the 5G position. NC requested that Material Testing Service (MTS) prepare a macrograph and a micrograph to determine the actual amount of OD concavity that was present on the profile of a fusion weld of 2-inch pipe with a wall thickness of 2.7mm (0.110 inches). This wall thickness, which is considered to be heavy-walled for fusion welding, especially for titanium, exhibited a concavity that, at the deepest point, was found to be 0.15mm. This is only 5.5% of the pipe wall thickness and this amount of concavity was agreed upon to be acceptable by both the testing agency and by NC.

Welding specifications for the welding of titanium are not up-to-date and do not address the issue of orbital welding. Weld Procedure Qualification was done, insofar as possible, according to ASME Section IX of the Boiler and Pressure Vessel Code and the Norwegian specifications for welding titanium.

This required bend and tensile tests in which the welds were sectioned and pulled to test whether the mechanical strength and ductility of the base metal had been compromised by welding. The test weld broke in the HAZ but the break occurred above the minimum tensile strength specified for the base material, which was acceptable. Radiographic examination of 100% of the qualification welds was done, but during production this was reduced to 20% of the welds except when the pressure specification was for greater than 40 Bar which required 100% radiography. NDT was done in-house by NC, but other testing was done by MTS. Tests included Vickers hardness testing, dye penetrant examination, and accelerated corrosion tests. Results of the Vickers hardness tests indicated that a light straw color oxidation of the welds was not detrimental to the mechanical properties, but that a blue discoloration was an indication of embrittlement. (See Table 1.)

NC is part of a technology ring in Norway which was established in the 1970's for the sharing of information and experience in the welding of titanium. All of the major oil companies participate in this group and are presently working on a specification for welding titanium that will be acceptable to all of the participating companies. In this way, the larger companies with more extensive experience with titanium are in a position to advise the smaller companies whose experience with this metal may be more limited, but who wish to take advantage of its favorable properties.

At about the half-way point of the platform construction project in June of 1993, NC had completed approximately 1,000 titanium welds on the Troll and Heidrun platforms. After welding titanium for over a year NC had passed the trial and error period and welding had become routine. NC's confidence in their ability to weld titanium was high. NC began by doing as much mechanized welding as possible and gradually reduced the percentage of welds being done manually. By the middle of 1993, there was about a 3% repair rate on manual welds with a gradual reduction in repair rate, and NC projected that this would be reduced to about 1-1/2%.

Although titanium has a reputation for being difficult to weld, they have found that if they respect the material, give adequate attention to cleanliness, preparation and proper purging, and provide consistent control of weld parameters, they can then successfully weld titanium on a routine basis. They feel that they have come a long way in their understanding of titanium since they started welding in the Spring of 1992.